.TH std::compare_partial_order_fallback 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::compare_partial_order_fallback \- std::compare_partial_order_fallback

.SH Synopsis
   Defined in header <compare>
   inline namespace /* unspecified */ {

       inline constexpr /* unspecified */                                 \fI(since C++20)\fP
           compare_partial_order_fallback = /* unspecified */;

   }
   Call signature
   template< class T, class U >

       requires /* see below */
   constexpr std::partial_ordering                                        \fI(since C++20)\fP

       compare_partial_order_fallback( T&& t, U&& u ) noexcept(/* see
   below */);

   Performs three-way comparison on subexpressions t and u and produces a result of
   type std::partial_ordering, even if the operator <=> is unavailable.

   If std::decay_t<T> and std::decay_t<U>> are the same type,
   std::compare_partial_order_fallback(t, u) is expression-equivalent to:

     * std::partial_order(t, u), if it is a well-formed expression;
     * otherwise, t == u ? std::partial_ordering::equivalent :
       t < u  ? std::partial_ordering::less :
       u < t  ? std::partial_ordering::greater :
                std::partial_ordering::unordered

   if the expressions t == u, t < u and u < t are all well-formed and each of
   decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u
   are evaluated only once.

   In all other cases, std::compare_partial_order_fallback(t, u) is ill-formed, which
   can result in substitution failure when it appears in the immediate context of a
   template instantiation.

.SH Example

    This section is incomplete
    Reason: no example

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR     Applied to         Behavior as published              Correct behavior
   LWG 2114  C++20      the fallback mechanism only required   constraints strengthened
   (P2167R3)            return types to be convertible to bool
   LWG 3465  C++20      the fallback mechanism did not require required
                        u < t to be well-formed

.SH See also

   partial_order performs 3-way comparison and produces a result of type
   (C++20)       std::partial_ordering
                 (customization point object)

.SH Category:
     * Todo no example
